超音波の著しい作用効果を電極反応に反映させることを目的とし,様々な超音波照射電解反応系が精査され,超音波照射により電極反応の高電流効率化ならびに/あるいは生成物選択性の向上が達成されることが示されている。また,これらの超音波効果は主として電極反応における物質移動過程への影響が顕著であることが理論と実験により検証され,特にキャビテーション現象がその主役を演じることが明らかにされている.さらに,超音波照射下における電気メッキや電解重合反応などの析出系では高密度で平滑な膜の形成が示されている.これらの成果は,電極反応の新しい制御手法としての超音波効果利用の第一歩として意義づけられ,今後の多様な展開が期待される.

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Sonochamical reaction provides a promising process to generate a novel reaction field, but lack of reproducibility among different experimental apparatus has been pointed out. One of the reasons of this problem comes from ambiguity of the sound filed in the reactor cell. The present paper shows how sound filed is generated in a reactor cell by the irradiation of ultrasound and how it changes depending on experimental conditions comparing optically visualized sound fields and sonoluminescence which gives a measure of reaction field in the cell. Theoretical analysis of the propagation of ultrasound in the cell proposes a method to generate a sound field effectively in the cell. In sonochemistry, two types of reactors are used; one is a standing-wave type reactor such as utilization of ultrasonic cleaner, and the other is a progressive-wave type reactor inserting an ultrasonic transducer horn into the liquid. The differences in the effects of ultrasonic irradiation between these reactors are aisb discussed.

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Refractory organic pollutants in aqueous solution are decomposed by ultrasonic irradiation. For the wastewater treatment, the gaslift bubble column sonochemical reactor was proposed and the performance was examined. The bubble column and draft tube were made of transparent acrylic resin and the inside diameter and height of column were 0.100 m and 2.00 m, respectively. The length of draft tube was 1.50 m and inside diameter was 0.055 m. Two ring spargers, which had 8 holes with 0.5 mm in diameter, and the porous sparger, whose pore diameter was 10 μm, were attached at the lower end of draft tube. One was for sparging gas into the draft tube (DT-BC [in]) and the other was for sparging gas into the annulus (DT-BC [an]) between column and draft tube. The planar PZT oscillator was attached at the center of reactor bottom and operated at 85 W. The standard bubble column (SBC) was used to compare with DT-BC. The sample to be decomposed was tetraphenylporphine tetrasulfonic acid aqueous solution. The initial concentration was 3.3×10^<-3> mol/m^3 and the sample volume was 13.5 L. For all modes of sparging gas, the apparent reaction rate constant increases with increasing gas velocity because of the enhancement of liquid mixing. To compare the results, the apparent reaction rate constant was plotted against the liquid mixing time. The liquid mixing time indicates the degree of mixing. Regardless of the modes of sparging gas, the apparent reaction rate constant increases linearly with decreasing liquid mixing time. The dependence of reaction rate constant on liquid mixing time in DT-BC is different from that in SBC. In the case of bubbling by the porous sparger, the apparent reaction rate constant is higher than that for the ring sparger. The sparge of microbubbles in the sonochemical reactor is considered to enhance the generation and collapse of the cavities.

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The equivalent circuit of sonoreactor is proposed, and the frequency dependence of H_2O_2 generation from distilled water by a sonochemical reaction was detected experimentally. The results are in good agreement with previous experimental results, which indicate that in sonochemical reactions, frequencies higher than 90kHz are more effective than frequencies of several ten kHz.

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The discoloration time of aqueous phenolphthalein solutions under sonication was examined. The Sonoreactor (HUS-1) which made in HON DA Electron Co. Ltd. was used. Ultrasonic frequencies of 44.0, 92.6, 127.3, 140.5, 518.0, and 1066. 0kHz were used. The pH-value was determined by the PH/ISE of ORION Co. Ltd., at 25℃ under the nitrogen atmosphere. The concentration ranges of phenolphthalein solutions were from 1.57xlO^<-5> to 9.42x10^<-5> mol/l. All the experimental were carried out at 25℃. The discoloration time depended on the concentration of phenolphthalein and the pH value of solutions. The discoloration time was correlated to the ultrasonic energy consumed in a reaction vessel, which is estimated on the basis of decomposition reaction of porphyrin. From these results, it was indicated that aqueous solution of phenolphthalein is useful for simple quantification of ultrasonic intensity for practical use.

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1997年に、Weningerら[Phys. Rev. E56, 745(1997)]が報告した金属板上の半球気泡からのソノルミネッセンスを、筆者が構築した気泡収縮の準断熱圧縮モデル[K. Yasui, Phys. Rev. E56, 6750(1997)]で理論的に解析した。Weningerらは、半球気泡からのソノルミネッセンスは、キセノンとクリプトン、すなわち質量数の大きい希ガスからでしか観測できなかったと報告した。本研究は、そのメカニズムを解明することを目的とする。本研究により、次のことが明らかになった。まず、質量数の大きい希ガスの方が、気泡内の最高到達温度が大きくなること。これは、質量数の大きい希ガスの方が熱伝導率が小さく、また気泡収縮の際に気泡内の気体のもつ運動エネルギーが大きく収縮が止まった際にそれが熱に変わるためである。質量数の大きい希ガスからの半球気泡ソノルミネッセンスが明るいのは、この気泡内の最高到達温度が大きいことと、イオン化エネルギーが小さいためである。イオン化エネルギーが小さいと、電離が起きやすくなり、発光の素過程である電子の制動放射と、電子と陽イオンとの間の放射性再結合が起きやすくなる。また、二原子分子の場合は、その比熱が大きいために、気泡内の最高到達温度が著しく小さくなる。その結果、発光強度も小さくなる。

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The effects of surfactant additives on the intensity of both sonoluminescence and sonochemical luminescence have been investigated. The surfactants are a cationic one of cetyltrimethylammonium bromide (CTAB) and an anionic one of sodium dodecyl sulfate (SDS). The intensity of luminescence was measured at various concentrations of the two surfactants while changing the driving frequency of ultrasound. In the intensity of sonoluminescence from distilled water, additives of the two surfactants had similar effects. At higher concentrations than the critical micelle concentration, the intensity was increased at a specific frequency of 129 kHz but decreased in the other frequency range. In the intensity of sonochemical luminescence from the luminol solution, quite different behavior between the two surfactants was observed. In the presence of 10 mM SDS, sonochemical luminescence due to chemical reaction of luminol was entirely shut out. Possible mechanisms resposible for the different behavior in the sonochemical luminescence are discussed.

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Dynamics of a sonoluminescing bubble was observed using a telemicroscope equipped with a video camera and a video recorder. The time resolution was 1/30 seconds, which was far slower than the period of oscillation, i.e., about 30 μs, and yet the observation gave some useful information, in particular, with a help of dust particles of a few μm in diameter which were used as tracers. First, the dust particles, drifting along with a stream, were seen to be scattered by the sonoluminescing bubble. Second, a few drifting dust particles were also seen to be trapped by the bubble, subsequently orbiting around it. The whole behavior suggests that the bubble may eject quadrupolar flows as a jet. The fact that the dust particles can be trapped by the bubble leads to a proposal that the bubble might be used as a micro reactor, where reactants are delivered in micro capsules and forming a dense region surrounding the bubble. This approach has an advantage that the medium, usually water, is little affected acoustically by adding reactants. The plausibility of the approach depends on whether or not there exists the material that is to be broken by Shockwaves emanating from the bubble at the moment of implosion.

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We have studied the efficiency of sonochemistry on the various sound field form. We defined the efficiency of sonochemistry as the amount of chemical reaction / sound energy density. We assume that efficiency is determined by the number and intensity of the cavitation bubbles. We directly measured the number of cavitation bubbles using CCD video camera. Sonochemically active bubbles are invisible therefore we counted visible ones. Although these are inefficient in chemical reactions, we assume that their number is proportional to the active ones. We found that the number of visible bubbles is proportional to sound intensity and the luminescence is also proportional to sound intensity. Thus, the number of bubble has a stronger effect on the sonochemical reaction than the intensity of each bubbles. The nucleus of bubbles is growing sonochemically active ones with sonication time. The growth time of the bubbles was measured by sonochemical luminescence and acoustic emission method. The time difference between measured value of luminescence and acoustic emission was about 0.1msec. The generation time was from 50msec to 1sec. High concentration of O_2 accelerates the growth of bubbles. The luminescence was increasing with the sonication time and reaches its saturation level which is independent of the O_2 concentration.

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The fluorescence spectra of 0.1 mM-luminol were measured in various pH's solutions by a fluorescence photometer The fluorescence emission peak was found at 450 nm and the excited peak was at 391 nm in lower pH's (pH < 6.5) solutions. The emission spectra were broadening and were red-shifted in the higher pH's (pH > 6.5) solutions although the excited spectra were not large changes. The fluorescence intensities were large changes dependent on the pH values of the solutions. It was found that the pKa_1^* value was 6.5 and the pKa_2^* value was estimated as 10.4 from the pH-dependent curves of the luminol (dibasic acid). The multibubble sonoluminescence from aqueous solutions containing luminol were observed in different pH's (pH =1.3 and pH = 10.6) solutions. Although there were no large changes of the spectra of different ultrasound frequencies (100 - 150 kHz) at 300 mV^<p-p>, the luminescence intensities at 141 kHz were changed in pH 1.3 or pH 10.6 solutions. The intensity in pH 10.6 solution was 215 times larger than that in pH 1.3 solution. It was considered that the intermediated state was mono-ionic form of diazo-quinone in pH's >10.4 (pKa_2^*) solutions. Furthermore, it was estimated the neutral form of luminol in lower pH's (< 6.5 = pKa_1^*) solutions. It was found the active ionic form of luminol sonoluminescence from the fluorescence pH-dependent curves (1.3 < pH < 10.6).

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Ultrasonic cavitation is well known to be applicable to the indecomposable waste such as chlorinated hydrocarbons on the waste treatment. In order to apply the sonochemical method to the treatment of wastewater including soil, the effects of particle properties on the sonochemical decomposition is experimentally investigated. The ultrasonic frequency was 22.8 kHz. The sample to be decomposed was porphyrin in aqueous solution. The dimensions of reactor were 110 mm in diameter and 140 mm in height. The volumes of samples were changed between 960 x 10'3 m3. The particles used in this study were spherical silica and alumina ones. The particle concentration and the particle diameter were changed. The sample was tetraphenylporphine tetrasulfonic acid aqueous solution. The porphyrin concentration in the sample was determined by using a UV spectrometer. As a measure of liquid mixing behavior, the liquid mixing time in the reactor was measured. Under ultrasonic irradiation, the recirculation of the particles enhances the liquid mixing in the reactor and the decomposition of porphyrin. Under the presence of particles the decomposition of porphyrin by ultrasonic irradiation proceeds at the first order reaction as in the case without particle. The decomposition reaction rate constant depends on the concentration and material of particles. It is considered that silica particles enhance the formation of cavities.

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Various epoxy flavanes substituted with methyl and methoxy groups were reacted with silica gel (230-400 mesh) under ultrasonic irradiation(Sus-4005C, 50W, 40kHz) for 1h to give the corresponding 3-hydoxyflavanes in good yields(ca.80-98%). When epoxy 3- hydroxyflavanes were reacted with silica gel only with stirring at 40 C in 12h, the yields were in ca.50-70 %. Epoxy flavanes were synthesized by oxidation of 2-cinnamyl substituted phenols with m- CPBA in methylene chloride. Epoxy flavanes were synthesized from the corresponding substituted phenols and cinnamyl alcohol with zinc chloride in benzene under reflux for 24h. For the formation of C-ring of flavane from epoxy compound, this ultrasonic method was useful in comparing with stirring only.

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Sonochemical reaction of water under various noble gases, such as Xe, Kr, Ar, and He, has been investigated by the determination of decomposition products. In Xe-, Kr- and Arsaturated solution, decomposition products of water, H_2, H_2O?2 and O_2, are observed while in He-saturated solution, H_2O_2 is observed but H_2 and O_2 are not. It is considered that more effective chemical reaction is derived during the collapse of cavitation bubbles containing noble gases of lower thermal conductivity and higher solubility in water. The yield of decomposition products is highest under Xe, which has the lowest thermal conductivity and highest solubility in water. In the cases of sonolysis of t-BuOH, the pyrolysis temperatures within the cavitation bubble are estimated from the product ratio of ethylene plus acetylene to ethane. The yields of decomposition product, methane, ethane, ethylene, and acetylene, are increased in order Xe > Ar > He, while the temperatures estimated from the product ratio under He, Ar and Xe are almost same in the range from about 3400 to 3900K. It is supposed that the acceleration of sonochemical reaction to vary dissolved gases is responsible for the gaseous characteristics of solubility in water. The number of cavitation bubbles is increased with increasing solubility. As a result, the reaction sites are increased. It is reported that the rapid compression of gas during cavitational collapse leads to nearly adiabatic heating of the contents of the bubble because thermal transport is slower than the collapse.

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Noble metal ions were reduced to form metallic nano-particles in diisooctyl sodium sulfosuccinate (AOT) by a sonochemical method. AOT has been known to form reverse micelle in organic solvent. Reduction rate of Pt(II) in AOT / dodecane solution was very slow with rate of 1.5 μM min^<-1> compared with those (ca. 20 μM min^<-l>) in aqueous solution of usual surfactants, such as polyethylene glycol monostearate (PEG-MS; noniomc), sodium dodecylbenzensulfonate (DBS; anionic) and sodium dodecylsulfate (SDS; anionic). In Pd(II) reduction, similar tendency to Pt(II) reduction was observed. A colloidal dispersion of Pt nano-particles in AOT / dodecane was stable for several months. The size of Pt particles (about 1 nm) prepared in this system was smaller than in SDS (3 nm). Au(III) ions with AOT in dodecane were reduced faster than Pt(II) and Pd(II), but formed particles (about 80 nm) were unstable to aggregate and precipitated in several hours. The results obtained in this work suggest that the reduction site of noble metal ions in AOT / dodecane is different from that in usual aqueous surfactants.

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Pd nanoparticles dispersed on alumina were prepared via the sonochemical reduction of tetrachloropalladate(II) (Pd(II)) in an aqueous solution. The reduction of Pd(II) to metallic Pd successfully proceeded even in the presence of alumina powder. The rates of Pd(II) reduction were dependent on the type of alcohol additive, which acts as an effective accelerator for Pd(II) reduction. The size of sonochemically formed Pd nanoparticles could be controlled by selecting the type of alcohol additive and the amount of coexisting alumina. Based on the analyses of the irradiated solution and the resulting powder by UV-VIS, pH and TEM, the formation mechanism of the supported materials was proposed. The catalytic activities of the prepared materials were investigated for the hydrogenation of olefins at ambient temperature. The rates of the hydrogenation were much higher than those of a conventionally prepared Pd/alumina catalyst and a commercial Pd black one. The activities and selectivities were also dependent on the catalysts prepared by different methods. In addition, Au/Pd nanoparticles supported on silica were also prepared by two successive processes composed of sonochemical reduction and sol-gel method. Characterization by TEM, XRD and UV-VIS, etc. and catalytic activities of these materials were investigated.

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Extensive studies in our laboratory have revealed that the sonochemical switching from ionic to radial in the nature of the reaction of lead tetraacetate with styrenes in acetic acid stems from the possible sonochemical excitation of styrenes in the cavitation process. We have found that the yields of the radical reactions of substituted styrenes show a linear relationship with their vapor pressures. In order to establish the hypothesis that the sonochemical activation depends on the vapor pressures of the substrates under the particular reaction conditions, the sonochemical activation of various alkenes including substituted styrenes was analyzed by determining the decomposition of pyrocatechol, a radical scavenger, in an acetic acid solution in the absence of lead tetraacetate. Results will be discussed from the viewpoint mentioned above.

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Electroactive films of aromatic polymers formed electrooxidatively on anodes have received much interest from not only fundamental but also practical aspects, since they exhibit peculiar properties such as electroconductivity, semiconductivity, electrochromism, etc. Generally, properties of polymers are originated from not only their chemical structures but also physical ones. However, few methods have been developed for controlling the physical structures of the polymer films so far. Mechanical energies can not derive chemical reactions but control. From this point of view, we aimed to control the structure of the films by applying mechanical energies such as ultrasound and centrifugal acceleration. By this way, we succeeded in preparing polyaniline films having unique properties. For instance, the density of the polyaniline film formed electrooxidatively on anodes was greatly affected by ultrasonic cavitation, that is, the film prepared under ultrasonic irradiation with a power higher than the cavitation threshold has a high density (About 10 times as high as an ordinary one). On the other hand, the morphological structure and electroconductivity of the film were influenced by the centrifugal field. The effects occurred anisotropically with regard to direction of the acceleration force to the working electrode surface. From this series of study, it was stated that mechanical energies such as ultrasound and centrifugal acceleration offer a useful method for controlling the structures and properties of the polyaniline films formed electrooxidatively on anodes.

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We have studied the polymerization of water-soluble monomers by ultrasound. The monomers studied were acrylic acid and N-isopropyl acrylamide. Water solutions of these monomers were prepared at various concentrations, and irradiated by ultrasound of 1 MHz or 500 kHz. In the case of acrylic acid, poly(acrylic acid) was obtained at the lowest concentration of the monomer (5 vol%), while gellation was observed at the higher concentrations (10〜50 vol%). At still higher concentrations, no cavitation was observed, and no polymerization took place. In the case of 5 vol% solution, Mw increased up to the sonication time of 120 min, and then gradually decreased because polymer chains were cut by ultrasound. After 180 min of sonication, more than 40 % of the monomers were polymerized. In the case of N-isopropyl acrylamide, poly(N-isopropyl acrylamide) was obtained at all the concentrations studied (2.5, 5 and 10wt%). In the case of 2.5 wt% solution, Mw was larger than that of the higher concentration solutions, but started to decrease at very short sonication time (20 min or less). After the sonication of 180 min, all the solutions gave relatively similar Mw.

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Effect of ultrasound on living radical polymerization of polystyrene was studied at the frequency of 27.5kHz. All the experiments were carried out under N_2 atmosphere at the constant of the ultrasonic intensity. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) was used as the nitroxide stable free radical to reversibly terminate the propagating chains. The average-number molecular weights and polydispersity were evaluated by the gel permeation chromatograph. The induction period of polymerization under sonication decreased greater than that of thermal polymerization without sonication. Ultrasound irradiation made the rate of polymerization increase. The number-weight molecular weight increased with increasing the reaction time under sonication as observed in usual living free radical polymerization. The polydispersity obtained here was independent of the reaction time and was within the range from 1.3 to 1.4.

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In solution, power ultrasounds decompose H_2O and produce various. Ultrasounds were applied to removal dissolved NO by oxidizing it to the ion NO^-_3 by the action of OH radicals produced by ultrasonic irradiation. This method is free from the use any oxidizing agents. The purpose of this work is dual. One is to establish quantitative relations between ultrasonic irradiation conditions and the amount of NO_3^- formed by the irradiation. The other is to design an appropriate set-up and find out optimal ultrasonic irradiation condition from reaction engineering perspectives. In the first stage, oxidation of NO to NO^-_3 ion by application of ultrasound in solution was examined. In this stage, sample gas NO gas was diluted with N_2 with the NO concentration equal to 5000ppm, while the solvent was deionized-degassed water. The concentration of NO^-_3 ion increased with duration of ultrasonic irradiation, demonstrating that the ultrasonic irradiation promotes the production of NO^-_3 Next, we constructed a liquid-circulation type apparatus in which compartment for ultrasonic irradiation is separated from the gas-bubbling section. Ultrasonic irradiation effects at various irradiation time were studied by analyzing the ion concentration of the same sample solutions. Efficiency of ultrasonic irradiation is far higher for liquid circulation mode than for the case of no circulation. Effect of gas flux and rate of liquid circulation were measured. The amount of ion NO3 produced is proportional to the liquid circulating velocity in a qualitative way. Increase in the amount of ion is probably related to the elimination of tiny bubbles of NO existing in the irradiation compartment by strong liquid flow.

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The sonochemical degradation of chlorinated hydrocarbons such as 1,1,1-trichloroethane (H_3C-CCl_3), trichloroethylene (HClC = CCl_2) and tetrachloroethylene (Cl_2C = CCl_2) in aqueous solution was carried out using the batch and continuous flow reactor at an ultrasonic frequency of 100 kHz. The ultrasound output power (P_<output>) actually transmitted to the medium is calculated by a calorimetric method and is 62 W (intensity is 0.62 W cm^<-2> ) corresponding to a 44.3 % input power. In the batch experiment, the pseudo-first-order reaction rate constants were H3C - CCl_3 (8.74 × 10^<-2> min^<-1>), HClC = CCl_2 (6.53 × 10^<-2> min^<-1>) and Cl_2C = CCl_2 (8.36 × 10^<-2> min^<-1>). The experiment in the continuous flow reactor was performed in the range of flow rate from 7 to 30 × 10^<-3> L min^<-1>. The conversion rate at a steady state depended on the residence time of the chlorinated hydrocarbon in the reactor. The conversion rate predicted by a model was in fair agreement with that obtained by experiment. In the model, the postulated multi-stage reactor consists of a series of several reactors, and ultrasonic degradation simulated in a three-stage reactor showed good degradation efficiency.

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It is known that the bio-materials are not only physically droken but also chemically degraded by ultrasonic irradiation. In this report, we try to degrate organnic phosphate using ultrasound. As the result, the mineralization of organic phosphate proceeded and phosphate ion was produced. After two-hour of sonication, about ninty percent of phosphorus was ionized. On long time sonication, however, it was difficult to detect phosphate ion. The inhibition of analysis by H2O2 produced from water was thought. This technique should be applied to the quantitative analysis of phosphorus in organic phosphate, after a several problems were solved.

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Photocatalytic cleavage of water is very attractive from the viewpoint of solar energy conversion. Although there were a few success, it was difficult to decompose water stoichiometricaly and continuously. On the other hand, sonochemical reaction of water results in the production of hydrogen and hydrogen peroxide. Photocatalytic decomposition of H_2O_2 proceeds easily. So I attempt sonophotocatalytic cleavage of water which means a simultaneous irradiation of light and ultrasound to the reactant. By the sumultaneous irradiation, H2 and 02 were evolved in the ratio of nearly 2:1 after an induction period. A lot of cases, however, the ratio of products was different from the ideal ratio (2:1). In this paper, the effects of adding TiO2 powder to the sonochemical reaction system on the ratio of products was reported.

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Precipitation of lead oxalate compound by mixing an aqueous solution of Pb(NO_3)_2 and an ethanolic solution of oxalic acid was performed in the absence or the presence of ultrasonication at 20-60℃ with addition of 1-10% ammonia water. The crystalline phases precipitated were PbC0_2O_4 (PC) and Pb_2C_2O_4(NO_3)_2・2H_2O (PCN). The ultrasonic effect on this precipitation is to promote the formation of PCN, which phase tends to be formed at lower temperatures, at lower pH, and at longer aging time. In addition, novel aspect of ultrasonic treatment for ceramic powder processing is proposed; i.e., the weak ultrasonication which is free from cavitation or sonochemistry might be applied to design the structure of the starting solutions for precise control of the precipitation process.

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Acoustic cavitation is a typical non-thermal phenomenon induced in liquids and biotissues by the exposure of ultrasound. It will disturb transmitting ultrasonic energy and thus should be avoided between the ultrasonic source and the target. Moreover, because of the potential tissue damage through its chemical or mechanical effects, cavitation should especially be avoided in diagnostic application. Techniques for suppression of acoustic cavitation is needed to control the cavitational phenomena in such undesirable situations. Reducing agents, such as sulfites are sometimes used to suppress cavitation in water for ultrasound coagulation therapy. However, it seems that three are no reports on quantitative results about their effects. We developed an experimental system which is suitable for measuring the effects of sulfites on suppressing cavitation with an optical detector, (fig.2). The effect of sodium sulfite (1M) on suppressing cavitation was investigated in water saturated with air or nitrogen (fig.3). Sodium sulfite increased the cavitation threshold about 1.7 times in both air saturated and nitrogen saturated water. Considering to apply chemical agents for suppressing cavitation in human body, we investigated a lesstoxic reducing agent, sodium ascorbate (vitamin C). Sodium ascorbate was found to be more effective than sodium sulfite in suppressing cavitation. The cavitation threshold in water in the presence of sodium ascorbate (1M) was about 2.5 times higher than in the absence. (Fig. 4-a). Our findings indicate the possibility of chemical agents for suppressing cavitation not only in acoustic coupling fluid but also in vivo because the low toxicity of sodium ascorbate.

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Iodine formation from Kl-starch solutions and cell lysis of rat thymocytes in Ar- and N_2O-saturated aqueous solutions induced by ultrasound with various frequencies, 38 kHz, 0.5 MHz, 1 MHz and 2 MHz were examined. The iodine formation was observed in Ar-saturated solutions exposed to 38 kHz, 0.5 MHz, and 1 MHz but not to 2 MHz. In contrast, the cell lysis was observed in all frequencies. In N_2O-saturated cell suspensions, the cell lysis was observed only in the sample sonicated with 38 kHz. No iodine formation was observed in the other samples. The relative ratio of the chemical effect vs. the cell survival at 70% (an example of the physical effect) was the order of frequencies, 0.5 MHz>1.0 MHz>38 kHz>2.0 MHz. Apoptosis of thymocytes estimated by the flow cytometry and the assay of DNA fragmentation increased with time after the isolation. However, no modification on apoptosis of thymocytes by 0.5 MHz ultrasound was observed in both Ar and N_2O saturated solutions. Partial protection was observed on the cell lysis and the viability after sonication with 0.5 MHz in Ar-saturated solution containing a free radical scavenger, cysteamine. These results suggest that chemical effects of ultrasound are prominent at specific frequencies and free radicals due to ultrasonic cavitation affect partially the cell lysis and the loss of viability of rat thymocytes.

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